The invention relates to “active implantable medical devices” as defined by the Directive 90/385/EEC of 20 Jun. 1990 the Council of the European Communities. This definition may include implants to continuously monitor heart rhythm and deliver, if necessary, electrical stimulation or resynchronization pulses. This invention relates more specifically to cardiac pacing leads to be implanted in the heart coronary network to allow the stimulation of a left or right cavity, a ventricle or an atrium.
Unlike the right cavities, for which implanting endocardial leads via the right peripheral venous system is sufficient, the implantation of permanent leads in a cavity of the left heart involves significant risks, such as the risk of bubbles passing to the cerebral vasculature located downstream of the left ventricle. For this reason, when it is desirable to stimulate a left cavity, it is most often chosen not to introduce a lead into the cavity to pace, but rather in the coronary network, the lead being provided with an electrode that is applied against the epicardial wall of the left ventricle or of the left atrium. These leads thus stimulate the heart muscle via one or more point electrodes whose position is a function of the predefined trajectory of the cannulated vein. A lead of this type is, for example, the Situs LV model marketed by Sorin CRM (Clamart, France) and described in EP 0993840 A1 and its US counterpart U.S. Pat. No. 6,385,492 (Sorin CRM S.A.S., previously known as ELA Medical). The introduction of such a lead is via the coronary sinus, from its opening in the right atrium. The lead is then pushed and oriented along the network of coronary veins to the selected site. This procedure is very difficult due to the particularities of the venous system, including the gradual reduction in diameter of the veins as the lead progresses into the selected coronary vein. Once the target vein is reached, the surgeon is looking for a satisfying stimulation site, with a good electrical contact with the stimulation electrode against the epicardial tissue. Then, contact should be maintained despite various variations or stresses over time.
A recent trend in the pacing of the left ventricle is the reduction of the diameter of the implantable portion in the coronary network, to a diameter of less than 4 French (1.33 mm), or even to less than 2 French (0.33 mm).
The size of the lead body is indeed a factor directly related to the controlled guiding capacity of the lead in the coronary venous system in order to be able to select specific stimulation sites located in certain collateral veins.
A lead is described in EP 2455131 A1 and US 20120130464 (Sorin CRM). It includes a lead body with a hollow sheath in which a microcable having a diameter of about 0.5 to 2 French (0.17 to 0.66 mm) slides. The micro cable can emerge to a length of 1 to 200 mm beyond the outlet of the lead body. This microcable, which forms the active part of the lead, has a plurality of exposed portions forming a succession of individual electrodes. The electrodes together form a network connected in series to increase the stimulation points in a deep area of the coronary network.
Its very small diameter allows the introduction of the microcable in a first vein (“go” vein) and then to a second anastomosis vein (“back” vein) ascending therein. There is a very frequent presence of distal anastomosis in the coronary venous system. In other words, at the end of some veins there can be passage to another vein, with the possibility of communication between two separate veins at the anastomosis via their respective distal ends. It thus becomes possible, with a single lead, to simultaneously stimulate two relatively remote areas located in two separate veins. The double effect of both the distance of the two areas and of the multiplication of points of stimulation in each zone can provide a particularly beneficial effect for the resynchronization of the functioning of the heart.
Another advantage of the small diameter of the active part of the lead is that it avoids the obstruction of a portion of the blood flow in the vein, which would lead to a deficiency of irrigation of the venous system downstream of the lead end.
Reducing the diameter of the lead, however, is not without drawbacks. Indeed, when the diameter of the lead is significantly lower than that in the vein, it can be difficult to ensure continuous contact with the electrodes. The exposed portion of the microcable, which forms an electrode, can be located in an intermediate, “floating” position in the middle of the vein, resulting in the contact points between the microcable and the wall of the vein being made in electrically isolated areas. Moreover, even in case of actual contact between the electrodes and the vein wall, this configuration may not be stable, because of a permanent heartbeat.
This may be particularly true in the case of microcables inserted through an anastomosis. If the veins are of small diameter in the region of the anastomosis, typically less than 1 French (0.33 mm), beyond the anastomosis they may join the coronary sinus after having passed the left ventricle, and the vein diameter increases. The very thin microcable, allowing to cross the anastomosis, may then move into a region of relatively large diameter, resulting in difficulty establishing a stable contact between the electrodes and the wall of the vein in the region.
One problem of the invention is to propose a structure of microlead ensuring continuous contact of all electrodes with the coronary veins, fixing the position of the lead in order to sustain the effectiveness of the stimulation. Another problem is the risk of displacement of the active part of the lead after it was implanted.
EP 2455131 A1 cited above provides for a retaining mechanism, such as a helical relief formed on the end of the lead body near the end thereof, near the outlet where the microcable emerges. The end of the lead body thus has a locally increased diameter of about 7 French (2.33 mm) for the mechanical holding of the lead body into the vein.
FR 2801510 A1 (corresponding to U.S. Pat. No. 6,549,812 B1) describes another mechanism for holding the lead in the target vein. However, this holding mechanism has the disadvantage of large size and may partially block the passage of blood flow in the vein. Moreover, although it can provide good retention of the lead body, it does not protect the portion of the lead inserted into the deep venous system. In such a case the telescopic microcable may be displaced by large movements of the patient. For example when a patient raises his arm, such movement tends to elongate the superior vena cava, with a risk of local traction applied to the lead, which traction is transmitted to the distal region implanted in the coronary network. These movements generated by the human body are thus an additional challenge relating to displacement of electrodes placed on the stimulation area.